Typical press lines such as transfer presses and tandem press lines are provided with workpiece conveyors for transferring workpieces into and out of work stations in synchronization with a series of multiple press work operations.
As such workpiece conveyers, there has been proposed a workpiece conveyor (transfer feeder) for use in a transfer press in Japanese Patent Kokai Publication No. 6-262280 filed by the present applicant. The workpiece conveyor 200 disclosed in this publication comprises, as shown in FIG. 24, a pair of lift beams 201 disposed side by side, extending in a workpiece conveying direction T; a plurality of cross bar carriers 202 aligned at intervals in the workpiece conveying direction T, being movably supported by these lift beams 201; cross bars 203 each extending in a direction perpendicular to the workpiece conveying direction T between an opposed pair of cross bar carriers 202. With vacuum cups (not shown) attached to the cross bars 203, a workpiece (not shown) is held by adsorption and conveyed.
In the workpiece conveyor 200, the lift beams 201 are suspended from above through lift mechanisms each having a servo motor 204 as a driving source. The adjacent cross bar carriers 202 aligned side by side in the workpiece conveying direction T are connected by a coupling rod 205, and the most downstream cross bar carrier 202 is connected to the distal end of a cam lever 207 through a coupling rod 206. A feed cam 208 rotatable by a power extracted from the press body is in contact with the cam lever 207. The feed cam 208 is rotated while the lift beams 201 being vertically moved by activation of the servo motors 204, so that the cam lever 207 is oscillated, thereby reciprocating all the cross bar carriers 202 at the same time in the workpiece conveying direction T. In this way, the movements in the vertical direction and in the workpiece conveying direction T are combined into a synthetic movement to cause the motion of the cross bars 203, which allows the workpiece to be transferred between the dies of the adjacent presses.
Another workpiece conveyor has been proposed in German Patent Application No. 19851743. As shown in FIG. 25, the workpiece conveyor 210 disclosed in this publication is designed such that a pair of uprights 211 are disposed at the right and left when viewed in the workpiece conveying direction T, opposing to each other with a specified spacing, and a cross bar 213 is disposed in the space between the pair of uprights 211, the cross bar 213 having a plurality of vacuum cups 212 for retaining a workpiece (not shown) suspended therefrom during a delivery of the workpiece from an upstream work station to a downstream work station. The workpiece conveyor 210 has, in combination, an oscillation driving unit 214 for rocking the cross bar 213 and an up-and-down driving unit 215 for moving the cross bar 213 up and down.
The oscillation driving unit 214 comprises a pair of arms 216 for supporting both ends of the cross bar 213 being held perpendicularly to the workpiece conveying direction T; carriages 217 each of which is supported by its associated upright 211 so as to be freely movable in a vertical direction and pivotally supports the proximal end of its associated arm 216; carriages 218 each slidably supported by its associated carriage 217; servo motors 219 each attached to its associated carriage 218; and guide rods 220 each of which has a proximal end to which a power transmission shaft 220a is secured and a distal end which is pivotally attached to the substantial center of its associated arm 216, the power transmission shaft 220a being connected to the output shaft of its associated servo motor 219 through a power transmission mechanism composed of bevel gears etc. The oscillation driving unit 214 is designed such that the relative movement of the carriages 217 and 218 allows each guide rod 220 to pivot about the power transmission shaft 220a, being driven by the servo motor 219, and each arm 216 is oscillated around its pivotal point by torque from the guide rod 220.
The up-and-down driving unit 215 is composed of servo motors 221 and screw mechanisms 222. Each screw mechanism 222 is interposed between its associated servo motor 221 and carriage 218, for converting the torque of the servo motor 221 into a linear motion force. In the up-and-down driving unit 215, the carriages 218 are moved up and down by activation of the servo motors 221 to move the entire oscillation driving unit 214 upward and downward. The up-and-down driving unit 215 has air-type balance cylinders 223 for imparting an upward energizing force to the carriages 218.
Changes in the height of the cross bar 213 caused by the oscillation of the arms 216 driven by the servo motors 219 are substantially compensated by the upward/downward movement caused by the servo motors 221, and the cross bar 213 is arbitrarily moved upward and downward in some cases, so that the motion indicated by broken line M in FIG. 25 is made.
The above-cited German Patent Application No. 19851743 has proposed a workpiece conveyor 230 as another embodiment, which comprises, as shown in FIG. 26, a pair of arms 232 pivotally supported by a pair of brackets 231 at their proximal ends, the brackets 231 being fixed to the bed located between two work stations so as to be spaced at a specified distance in a direction perpendicular to the workpiece conveying direction T. Pivotally attached to the distal ends of the pair of arms 232 is the cross bar 213 which is laterally held so as to extend in a direction perpendicular to the workpiece conveying direction T. In the workpiece conveyor 230 of this embodiment, each of the pair of arms 232 is oscillated around its pivotal point located at the proximal end thereof by activation of an oscillation driving unit 233 attached to each bracket 231. Each arm 232 is of a nested-structure and designed to be expanded and contracted in a longitudinal direction by an expansion/contraction driving unit (not shown) housed in the arm 232. The workpiece conveyor 230 includes a horizontal condition keeping system (not shown) for keeping a workpiece (not shown) in a horizontal condition, and this horizontal condition keeping system has first toothed pulleys secured to the cross bar 213; second toothed pulleys mounted at the proximal ends of the arms 232; and a toothed belt wound around each pair of first and second toothed pulleys. By unrotatably fixing the second toothed pulleys, the workpiece can be maintained in a horizontal condition even when the pair of arms 232 oscillate.
Changes in the height of the cross bar 213 caused by the oscillation of the arms 232 driven by the oscillation driving unit 233 are substantially compensated by proper expansion/contraction of the arms 232 and the cross bar 213 is arbitrarily moved upward and downward in some cases, whereby a motion similar to the motion M (See FIG. 25) of the workpiece conveyor 210 described earlier can be obtained.
The robot type and the loader/unloader type have been known as workpiece conveyors for use in a tandem press line. In the robot type workpiece conveyors, an articulated handling robot is placed between every adjacent pair of press machines, and a workpiece is carried out of a preceding press work station and carried into a succeeding press work station, using the handling robots. The robot type workpiece conveyors have the advantage of setting the trajectory of carrying-out and carrying-in of a workpiece in conformity to the dies. In contrast with this, the loader/unloader type workpiece conveyors are designed such that a loader and unloader of a link structure are provided for the upstream side face and downstream side face, respectively, of each press machine and a shuttle carriage is provided between the upstream unloader and the downstream loader. Carrying-out and carrying-in of a workpiece with respect to the press machine body are done by the unloader and the loader respectively and the delivery of the workpiece to the next station is done by the shuttle carriage.
Regarding the above-described conventional workpiece conveyors for a transfer press, the workpiece conveyor 200 disclosed in Japanese Patent Kokai Publication No. 6-262280 is designed to drive all the cross bar carriers 202 at the same time by the driving means composed of the cam mechanism and link mechanism and therefore the driving system becomes heavy, thick, long, bulky and complex, resulting in poor visibility. In addition, the rigidity of the lift beams 201 needs to be increased in order to ensure positioning accuracy, which unavoidably leads to an increase in the weight of the lift beams 201. For the upward and downward movement of the cross bars 202, it is necessary to entirely move the lift beams 201 up and down, so that large-sized servo motors are needed as the servo motors 204 and, in consequence, the cost as well as the size of the whole system is inevitably increased. Further, since the motions of the cross bars 203 with respect to the workpiece conveying direction T at all the work stations are dependent on the feed cam 208, restrains are placed on setting of the optimum motion pattern for each work station, accompanied with a drop in the degree of freedom.
The workpiece conveyor 210 of German Patent Application No. 19851743 has revealed such a problem that the oscillation driving unit 214 and its peripherals are complex in structure. Additionally, the workpiece conveyor 210 cannot be installed in places where the uprights 211 are not provided and therefore its installation place is limited. Although the size of the servo motors 221 is reduced by the provision of the balance cylinder 223, complicated construction is unavoidably involved.
In the workpiece conveyor 230 according to the second embodiment of German Patent Application No. 19851743, although the cross bar 213 is attached to the distal ends of the pair of expandable/contractible arms 232 that are driven to oscillate along the workpiece conveying direction T and the relative distance between the cross bar 213 and each arm 232 is varied by the expansion and contraction of the pair of arms 232, the minimum length (the length when the arms 232 are fully contracted) of the arms 232 that is inevitably dependent of their structure is relatively long because of the nested-structure of the arms 232, which leads to the problem that the zone within which the cross bar 213 can move is comparatively narrow. According to German Patent Application No. 19851743, the horizontal condition keeping system has the first toothed pulleys secured to the cross bar 213; the second toothed pulleys mounted on the proximal ends of the arms 232; and a toothed belt wound around each pair of first and second toothed pulleys, and even when the pair of arms 232 oscillate, the workpiece can be kept in a horizontal condition, thanks to the second toothed pulleys secured so as to be unrotatable. However, the structure, in which a toothed belt is wound around each pair of first and second toothed pulleys, has presented such a doubt that the pair of arms cannot be expanded and when the pair of arms are contracted, the toothed belt is slackened with the result that the system cannot function properly. No technique for clearing up such a doubt is described in German Patent Application No. 19851743.
Regarding the workpiece conveyors used in a tandem press line, the robot type workpiece conveyors present the problem that if adjacent press machines are apart a long distance, the arm length between the joints needs to be increased in proportion to the distance between the adjacent press machines and therefore the driving unit for each joint is required to have higher output power, which progressively increases the whole size of the workpiece conveyor. On the other hand, if the distance between the adjacent press machines is short, it becomes very difficult to set a workpiece trajectory so as to avoid interference between the workpiece and the uprights etc. In the case of the loader/unloader type workpiece conveyors, it is required to install a shuttle carriage between every adjacent pair of press machines, so that a large scale system and, in consequence, a large installation space are involved. In addition, since the workpiece needs to be transferred between the shuttle carriage and the work station, there is a possibility that a delivery error may occur. As described above, a large-scale system structure is unavoidably involved in any of the above conventional workpiece conveyors and therefore they have difficulties in achieving increased handling speed and productive efficiency.
As an attempt to solve the above problems, the present applicant has already proposed a workpiece conveyor for use in a tandem press line in the earlier invention (Japanese Patent Application No. 2001-400849), which provides high workpiece delivery speed, with relatively slim configuration. As illustrated in FIGS. 27(a), 27(b), the workpiece conveyor 240 of this earlier invention is designed to have lift beams 241 extending in the workpiece conveying direction T; carriers 242 and sub carriers 243 which are movable along the longitudinal direction of the lift beams 241; and a cross bar 245 with vacuum cups 244 disposed between the right and left pair of sub carriers 243 and serving as a workpiece holding means.
In the workpiece conveyor 240 of the earlier invention, the lift beams 241 are upwardly and downwardly moved by activation of lift axis servo motors 246, thereby upwardly and downwardly moving the vacuum cups 244 through the carriers 242, the sub carriers 243 and the cross bar 245. By moving the carriers 242 in the longitudinal direction of the lift beams 241 by activation of linear motors (not shown) each interposed between a lift beam 241 and a carrier 242 and making the sub carriers 243 offset in the moving direction of the carriers 242 by linear motors (not shown) each interposed between a carrier 242 and a sub carrier 243, the cross bar 245 and the vacuum cups 244 are moved in the workpiece conveying direction T. In this way, two orthogonal drive axis positions (more particularly, the position with respect to the vertical direction and/or the position with respect to the workpiece conveying direction T) are controlled, thereby controlling the moving trajectory of the vacuum cups 244, in other words, the trajectory of conveyance of the workpiece W.
The workpiece conveyor 240 of the earlier invention, however, has proved unsuccessful in overcoming the foregoing drawbacks. Specifically, the rigidity of the lift beams 241 needs to be increased in order to achieve high positioning accuracy, resulting in an increase in the size of the lift beams 241, and the lift beams 241 need to be entirely moved up and down when moving the cross bar 245 up and down so that large-sized servo motors are necessary as the servo motors 246, increasing the whole size of the system and the cost as a logical consequence. In addition, since the ends of each lift beam 241 are located within the zone of the die carry-in-and-out passage, replacement of the die is required to be carried out after the lift beams 241 are once lifted out of the die carry-in-and-out passage, which leads to poor productive efficiency.
The present invention is directed to overcoming the above shortcomings and a primary object of the invention is therefore to provide a workpiece conveyor for a press line, which has a simple small-sized structure thereby providing cost reduction, and offers an increased degree of freedom in setting a motion pattern and improved productive efficiency.